The present invention relates to a method for absorption and release of hydrogen where a hydrogen storage metal alloy is repeatedly subjected to pressurization and depressurization of hydrogen. In more detail, the present invention relates to a hydrogen storage metal alloy having a two-stage plateau- or inclined plateau-property. Particularly, the present invention relates to a method for absorption and release of hydrogen where the amount of released hydrogen increases within practical pressure ranges and temperature ranges, to a hydrogen storage metal alloy suitable for such a method for absorption and release of hydrogen and to a hydrogen fuel battery using the above method for absorption and release of hydrogen.
At present, there have been worries not only about acid rain due to an increasing NOx (nitrogen oxides) but also about global warming due to an increasing C02 in association with an increase in consumption of fossil fuel such as petroleum and such environmental destruction has become a serious problem. Therefore, our attention has been greatly concentrated on development and practical application of various kinds of clean energy which is friendly to the earth. Part of means for developing such a new energy is a practical application of hydrogen energy. Hydrogen is a constituent element of water inexhaustibly present on the earth and can be not only produced using various kinds of primary energy but also utilized as fluid energy in place of conventionally used petroleum without the risk of destroying the environment because its product of combustion is only water. In addition, unlike electricity, it has excellent characteristics such as its relatively easy storage.
In recent years, therefore, investigation has been actively conducted involving hydrogen storage metal alloys as media for storing and transporting hydrogen and their practical application has been expected. Such hydrogen storage metal alloys are metals/alloys which can absorb and release the hydrogen under an appropriate condition and, by the use of such alloys, it is possible to store the hydrogen not only at lower pressure but also at higher density as compared to the case of the conventional hydrogen cylinders. In addition, the hydrogen volume density thereof is nearly equal to or rather greater than that of liquid or solid hydrogen.
These hydrogen storage metal alloys which have been chiefly investigated are, for example, those alloys which each have a body-centered cubic (hereinafter, referred to as xe2x80x9cBCCxe2x80x9d) structure, including V, Nb, Ta or Crxe2x80x94Tixe2x80x94Mn alloys, Crxe2x80x94Tixe2x80x94V alloys, etc. as proposed in Japanese Unexamined Patent Publication (Kokai) No. 10-110225 (JP, A, 10-110225). It has been known that those alloys adsorb and store hydrogen in greater quantities as compared with AB5 alloys such as LaNi5 and AB2 alloys such as TiMn2 which have been practically used until now. This is because the number of hydrogen absorbing sites in the crystal lattice is great in the BCC structure and the hydrogen absorbing capacity is as large as H/M=ca. 2 wherein H is occluded hydrogen and M is a constituent element for the alloy (about 4.0 wt % in alloys of V, etc. having an atomic weight of around 50), being extremely large.
It has been known that such a BCC alloy having a relatively large hydrogen absorbing capacity conducts a two-step reaction during the course of its absorbing hydrogen to form a hydride, as shown in Reilly and R. H. Wiswall, Inorg. Chem., 9 (1970), 1678). For example, V reacts with hydrogen at ambient temperature and forms two kinds of hydrides depending upon the pressure of hydrogen. At first, at the initial reaction stage wherein hydrogen pressure is low, a very stable hydride is formed as Vxe2x86x92VH0.8 (xcex1 phasexe2x86x92xcex2 phase) (hereinafter, referred to as xe2x80x9clow-pressure plateau partxe2x80x9d) and, at around room temperature, a reverse reaction thereof rarely happens. When further more hydrogen pressure is applied, a hydride is formed as VH0.8xe2x86x92VH2.01(xcex2 phasexe2x86x92xcex3 phase; referred to as xe2x80x9chigh-pressure plateau partxe2x80x9d). The equilibrium hydrogen pressure of this reaction is appropriate (approximately a few atmospheric pressure at around room temperature). Therefore, such V-containing BCC alloys have been briskly studied as high-capacity hydrogen storage metal alloys.
FIG. 1 is a conceptional chart of a PCT curve of a single substance V having a two-stage plateau comprised of the aforementioned low-plateau and high-plateau parts. The flat region at the hydrogen pressure of 10xe2x88x921 Pa in FIG. 1 is a low-pressure plateau part and the flat region at the hydrogen pressure of 106 Pa is a high-pressure plateau part. The inclined region between the low-pressure plateau part and the high-pressure plateau part is a region complying with Sieverts""s law. Besides V, an example of the metal having such a two-stage plateau is Nb (low-pressure phase: NbH, high-pressure phase: NbH2). In addition, Ti shows a two-stage plateau by a transformation of xcex1xe2x86x92xcex2xe2x86x92xcex3 although it operates at elevated temperature. An intermetallic compound having a two-stage plateau includes FeTi which works at near 40xc2x0 C. Further, alloys such as (Zr, Ti)V2 show an inclined plateau and those alloys are also used as hydrogen storage metal alloys.
Examples of the prior art techniques presumably based upon the idea of developing a high-capacity hydrogen storage metal alloy relying on the above-mentioned two-stage plateau and inclined plateau characteristics are as follows:
(a) spinodal decomposition tissues are expressed in a body-centered cubic structure Ti alloy (the above JP, A, 10-110225);
(b) a Tixe2x80x94Crxe2x80x94V alloy is admixed with Cu and/or rareearth elements (the above JP, B2, 4-77061);
(c) a Ti alloy melt is rapidly cooled to form a BCC mono phase at room temperature (Japanese Unexamined Patent Publication (Kokai) No. 10-158755 (JP, A, 10-158755)); and
(d) a BCC alloy comprised as main elements of Tixe2x80x94Cr is adjusted for its lattice constant (Japanese Unexamined Patent Publication (Kokai) No. 07-252560 (JP, A, 07-252560)).
Among the above-mentioned methods for absorbing and releasing hydrogen, those where temperature for absorption and desorption of hydrogen is mentioned are JP, A, 10-110225 and JP, A, 07-252560, both which disclose the methods where hydrogen is absorbed and released at a constant temperature, provided that, in the latter JP, A, 07-252560, the activating pretreatment is carried out by means of a two-stage treatment comprising a low temperature in the former stage and a high temperature in the latter stage while the temperature for hydrogen absorption and desorption is constant (20xc2x0 C.). In Japanese Patent Publication No. 59/38293 (JP, B2, 59/38293), hydrogen is absorbed with a hexagonal Tixe2x80x94Crxe2x80x94V type alloy which is not a BCC alloy and a method of heating at 100xc2x0 C. or higher (lines 32 to 39, column 4) is for absorbing and releasing hydrogen at a constant temperature as well.
However, in the hydrogen storage metal alloy having the above-mentioned two-stage plateau characteristic such as V-containing BCC alloy which has been often investigated as the high-capacity hydrogen storage metal alloy, the hydrogen-absorbing reaction at the low-pressure plateau region proceeds only to the side of the reaction with hydrogen at room temperature. Therefore, it has not been carried out in the prior art that the hydrogen occluded is taken out in such a low-pressure plateau region and used as an effective hydrogen.
Thus, in the above-mentioned JP, A, 10-110225 and JP, B2, 4-77061, such a low-pressure plateau region is not referred to. In the latter patent, since there is a teaching that the production of TiH2 (high-pressure plateau region compound) is to be avoided, only the hydrogen-absorbing reaction between the low-pressure plateau region and the high-pressure plateau region is utilized.
It is said that, in general, the amount of hydrogen taken out from a body-centered cubic structure type hydrogen storage metal alloy such as pure V and pure Nb is very low as compared with the theoretical amount (Hydrogen Storage Metal Alloyxe2x80x94Physical Properties and Applications, New Edition, by Yasuaki Osumi, published by Agne Technique Center, Japan, first printing of the first edition issued on Oct. 30, 1993, page 309).
In AB5 alloys such as LaNi5 or BCC alloys, which have been practically utilized up to now, it is possible to control the equilibrium pressure regarding reaction with hydrogen by controlling the alloy components. It is also possible that the equilibrium pressure of the hydrogen storage metal alloy with hydrogen is controlled by the operating temperature. However, the conventional research on alloys as such is not particularly based on consciousness of improvement in the hydrogen-absorbing characteristic at the above-mentioned low-pressures plateau region.
Accordingly, it is believed that, in order to increase the hydrogen absorption capacity in the aforementioned BCC type hydrogen storage metal alloy, it is effective that the hydrogen in the reaction of a phasexe2x86x92xcex2 phase, i.e., the reaction at the low-pressure plateau part (for example, the reaction of Vxe2x86x92VH0.8 in the case of V), contributes to the reaction of absorption and desorption in addition to the xcex2-phase region of the BCC type alloy (a portion complying with a Sieverts""s law between a low-pressure plateau region and a high-pressure plateau region). However, such a means has not been disclosed yet.
Accordingly, an object of the present invention is, with regard to the conventional pure V or pure Nb showing a two-stage plateau or inclined plateau region or BCC solid solution alloys including not only solid solutions showing a hydrogen absorption/desorption reaction similar to the above-mentioned metal, but also Tixe2x80x94Cr system alloys, etc., to provide a hydrogen storage metal alloy in which the hydrogen not only between xcex1 phasexe2x86x92xcex2 phase, i.e. in the reaction at the low-pressure plateau region but also at a low-pressure xcex2 phase region (a low-pressure region showing a behavior similar to a Sieverts""s law between a low-pressure plateau region and a high-pressure plateau region) is made contributed in an absorption/desorption reaction of hydrogen in a reversible manner so that much more amounts of hydrogen can be absorbed and released and also to provide not only a method for absorbing and releasing hydrogen with the said alloy but also a hydrogen fuel battery using the said method.
In order to achieve the aforementioned objects, the present invention provides a novel hydrogen storage metal alloy. According to the present invention, the novel hydrogen storage metal alloy has the following characteristics:
(1) it has as its main phase a body-centered cubic structure-type phase exerting a two-stage or inclined plateau characteristic in a hydrogen storage amount vs hydrogen pressure relation, and
(2) the composition ratio of constituent metals for the alloy is adjusted to an appropriate range in order to reduce the stability of the hydrogen occluded in the alloy during the low-pressure plateau region or the lower plateau region of the inclined plateau such that an alloy temperature (T2) during at least a period in a hydrogen release process can be brought to higher than an alloy temperature (T1) in a hydrogen-absorption process (T2 greater than T1) whereby at least part of the occluded hydrogen will be made desorbable during the low-pressure plateau region in the above-mentioned two-stage plateau or the lower plateau region of the inclined plateau.
Such characteristics lead to the following:
the occluded hydrogen can be unstabilized in the alloy so that the alloy temperature may be brought to high (T2) during the hydrogen desorption process, thereby facilitating the release of hydrogen during the aforementioned low-pressure plateau region or the lower plateau region of the inclined plateau region, and therefore the occluded hydrogen at the low-pressure plateau region or the lower plateau region of the inclined plateau region, which has been neither desorbed nor utilized at all, can be taken out as utilizable hydrogen, with the result that the amount of the utilizable hydrogen in such a hydrogen storage metal alloy will be increased.
It is preferred that the hydrogen storage metal alloys of the present invention are those wherein the alloy temperature (T1) during the hydrogen-absorbing process may range from the extremely low temperature in the living areas on the earth to 373 K.
As a result thereof, the alloy temperature (T1) during the hydrogen-absorbing process can be made near an ambient temperature region whereby the practicability can be improved.
It is preferred that the hydrogen storage metal alloys of the present invention are V alloys which each not only have a suitably adjusted composition to reduce the stability of the occluded hydrogen as aforementioned but also contain 0 to 95 at % of at least one or more members selected from the group consisting of Nb, Ta, W, Mo, Ti, Cr, Mn, Fe, Al, B, Co, Cu, Ge, Ni and Si.
As a result thereof, the alloys each having such a composition are highly effective in unstabilizing the occluded hydrogen therein and therefore suitable for releasing a large amount of hydrogen therefrom during the low-pressure plateau region or the lower plateau region of the inclined plateau by raising the alloy temperature during a hydrogen-desorbing process.
It is preferred that the hydrogen storage metal alloys of the present invention are those alloys which each have not only a suitably adjusted composition to reduce the stability of the occluded hydrogen as aforementioned but also a fundamental composition of the formula:
VaTi(41xe2x88x920.4a+b)Cr(59xe2x88x920.6axe2x88x92b)
wherein 0xe2x89xa6axe2x89xa670 at % and xe2x88x9210xe2x89xa6bxe2x89xa610 at %.
As a result thereof, the alloys each having such a composition can occlude a large amount of hydrogen at the high-pressure plateau region and are greatly effective in unstabilizing the occluded hydrogen therein. Therefore, such alloys are preferable to release a large quantity of occluded hydrogen during the low-pressure plateau region or the lower plateau region of the inclined plateau by raising the alloy temperature during the hydrogen-desorbing process and have an effective amount of utilizable hydrogen in great quantities, thereby giving a high practicability.
It is preferred that the hydrogen storage metal alloys of the present invention are those alloys which each have not only a suitably adjusted composition to reduce the stability of the occluded hydrogen as aforementioned but also a fundamental composition of the formula:
V(axe2x88x92d)M2dTi(41xe2x88x920.4a+b)Cr(59xe2x88x920.6axe2x88x92bxe2x88x92c)Mc
wherein 0xe2x89xa6axe2x89xa670 at %, xe2x88x9210xe2x89xa6bxe2x89xa610+c, 0xe2x89xa6c, 0xe2x89xa6dxe2x89xa6a, M is at least one or more members selected from the group consisting of Nb, Mo, Ta, W, Mn, Fe, Al, B, C, Co, Cu, Ge, Ln (various lanthanoid metals), N, Ni, P and Si, and M2 is at least one or more members selected from the group consisting of Mo, Nb, Ta, W, Mn, Fe and Al.
As a result thereof, the alloys each having such a composition can occlude a large amount of hydrogen at the high-pressure plateau region and are greatly effective in unstabilizing the occluded hydrogen therein. Therefore, such alloys are preferable to release a large quantity of occluded hydrogen during the low-pressure plateau region or the lower plateau region of the inclined plateau by raising the alloy temperature during the hydrogen-desorbing process and have an effective amount of utilizable hydrogen in great quantities, thereby giving a high practicability. In addition, as a result of suitable admixture with at least one or more elements selected from the above-mentioned lanthanoid metals, N, Ni, P and Si, it is achievable to lower the melting point of the alloy and to improve the flatness of the plateau resulted thereby, and it is possible to either free the alloy of a heating treatment which is apt to cause a spinodal decomposition or shorten a heating treatment time, thereby leading to an effect that a decrease in the hydrogen storage amount can be suppressed.
It is preferred that the hydrogen storage metal alloys according to the present invention are those wherein the tissue structure of the above-mentioned suitably adjusted hydrogen storage metal alloy is of a body-centered cubic structure mono phase without any spinodal decomposition phase or has a body-centered cubic structure together with only a minimum spinodal decomposition phase which is unavoidably produced.
As a result thereof, the hydrogen storage metal alloy has a minimum spinodal decomposition phase or has no spinodal decomposition phase, thereby enabling a decrease in hydrogen adsorption capacity due to the formation of spinodal decomposition phase to be suppressed as little as possible.
A method for absorbing and releasing hydrogen by using the hydrogen storage metal alloy according to the present invention comprises:
applying repeatedly hydrogen pressurization and depressurization to the hydrogen storage metal alloy of a body-centered cubic structure-type phase exerting a two-stage or inclined plateau characteristic in a hydrogen storage amount vs hydrogen pressure relation in an appropriate fashion to absorb and release hydrogen, and at least at one stage during the release of hydrogen, making the temperature (T2) of the above-mentioned hydrogen storage metal alloy higher than the temperature (T1) of the hydrogen storage metal alloy during the hydrogen absorption process (T2 greater than T1).
Such characteristics lead to the following: it is now possible to take out as a utilizable hydrogen the occluded hydrogen at the low-pressure plateau region or the lower plateau region of the inclined plateau which has not been desorbed and utilized at all whereby the amount of utilizable hydrogen can be increased in the hydrogen storage metal alloy.
It is preferred that the alloy temperature (T1) during the above hydrogen-absorbing process is within a range of from the extremely low temperature in the living areas on the earth to 373 K in the method for absorbing and releasing hydrogen by using the hydrogen storage metal alloy according to the present invention.
As a result thereof, the alloy temperature (T1) during the hydrogen-absorbing process can be made near an ambient temperature region whereby the practicability can be improved.
It is preferred that the method for absorbing and releasing hydrogen according to the present invention comprises using a hydrogen storage metal alloy wherein the composition ratio of the constituent metals for the alloy is adjusted to an appropriate range in order to reduce the stability of the hydrogen occluded in the alloy during either the low-pressure plateau region or the lower plateau region of the inclined plateau such that the temperature of the said alloy can be brought to the above high temperature (T2) whereby at least part of the occluded hydrogen will be made desorbable during either the low-pressure plateau region in the above-mentioned two-stage plateau or the lower plateau region of the inclined plateau.
As a result thereof, the occluded hydrogen can be unstabilized in the alloy, thereby facilitating the release of hydrogen from either the above low-pressure plateau region or the lower plateau region of the inclined plateau when the temperature of the said alloy is made higher (T2) during the hydrogen release process, with the result that the amount of effective hydrogen can be increased.
It is preferred that the method for absorbing and releasing hydrogen according to the present invention comprises using a hydrogen storage metal alloy wherein the aforementioned adjustment is in such a manner that the composition ratio of the constituent metals for the alloy is adjusted suitably so as to reduce the stability of the occluded hydrogen in the alloy within either the low-pressure plateau region or the lower plateau region of the inclined plateau.
It is preferred that the method for absorbing and releasing hydrogen according to the present invention comprises using a hydrogen storage metal alloy with a suitably adjusted composition to reduce the stability of the above occluded hydrogen, said hydrogen storage metal alloy being a V alloy containing 0 to 95 at % of at least one or more members selected from the group consisting of Nb, Ta, W, Mo, Ti, Cr, Mn, Fe, Al, B, Co, Cu, Ge, Ni and Si.
As a result thereof, the alloy having such a composition is highly effective in unstabilizing the occluded hydrogen therein and therefore suitable for releasing a great deal of hydrogen from the low-pressure plateau region or the lower plateau region of the inclined plateau by raising the alloy temperature during the hydrogen release process.
It is preferred that the method for absorbing and releasing hydrogen according to the present invention comprises using a hydrogen storage metal alloy with a suitably adjusted composition to reduce the stability of the above occluded hydrogen, said hydrogen storage metal alloy having a fundamental composition of the formula:
xe2x80x83VaTi(41xe2x88x920.4+b)Cr(59xe2x88x920.6axe2x88x92b)
wherein 0xe2x89xa6axe2x89xa670 at % and xe2x88x9210xe2x89xa6bxe2x89xa610 at %.
As a result thereof, the alloy having such a composition has not only a great deal of occluded hydrogen therein at the high-pressure plateau region but also a high activity in unstabilizing the hydrogen occluded in the alloy. Therefore, such alloys are suitable for releasing a great deal of hydrogen from the low-pressure plateau region or the lower plateau region of inclined plateau by raising the alloy temperature during the hydrogen release process and highly practicable because a great amount of effective hydrogen is utilizable therein.
It is preferred that the method for absorbing and releasing hydrogen according to the present invention comprises using a hydrogen storage metal alloy with a suitably adjusted composition to reduce the stability of the above occluded hydrogen, said hydrogen storage metal alloy having a fundamental composition of the formula:
V(a+b)M2dTi(41xe2x88x920.4a+b)Mc
wherein 0xe2x89xa6axe2x89xa670 at %, xe2x88x9210xe2x89xa6bxe2x89xa610+c, 0xe2x89xa6c, 0xe2x89xa6dxe2x89xa6a, M is at least one or more members selected from the group consisting of Nb, Mo, Ta, W, Mn, Fe, Al, B, C, Co, Cu, Ge, Ln (various lanthanoid metals), N, Ni, P and Si, and M2 is at least one or more members selected from the group consisting of Mo, Nb, Ta, W, Fe and Al.
As a result thereof, the alloy having such a composition has not only a great deal of occluded hydrogen at the high-pressure plateau region but also a high activity in unstabilizing the hydrogen occluded in the alloy. Therefore, such alloys are suitable for releasing a great deal of hydrogen from the low-pressure plateau region or the lower plateau region of inclined plateau by raising the alloy temperature during the hydrogen release process and highly practicable because a great amount of effective hydrogen is utilizable therein.
In addition, the suitable admixture with at least one or more elements selected from the group consisting of the above-mentioned lanthanoid metals, N, Ni, P and Si leads to a decrease in the melting point of the alloy and an improvement in the flatness of the plateau resulted thereby whereupon the resultant alloy products are successful in suppressing a decrease in hydrogen adsorption capacity because a heating treatment which is apt to cause a spinodal decomposition is not applied or a treating time is shortened.
It is preferred that the method for absorbing and releasing hydrogen according to the present invention comprises using a hydrogen storage metal alloy wherein the tissue structure of the aforementioned suitably adjusted hydrogen storage metal alloy is of a body-centered cubic structure mono phase without any spinodal decomposition phase or has a body-centered cubic structure together with only a minimum spinodal decomposition phase which is unavoidably produced.
As a result thereof, the hydrogen storage metal alloy has a minimum spinodal decomposition phase or has no spinodal decomposition phase. Therefore, a reduction in the amount of occluded hydrogen by the formation of spinodal decomposition phase can be suppressed as little as possible.
The hydrogen fuel battery of the present invention is characterized in that the battery is equipped with
a hydrogen storage tank including a hydrogen storage metal alloy,
a temperature controlling means whereby the above hydrogen storage metal alloy is directly heated or cooled or the atmospheric temperature of the said hydrogen storage metal alloy is raised or cooled,
a fuel battery cell in which hydrogen supplied from the said hydrogen storage tank can be subjected to a chemical change to output an electric power, and
a controller where a control is done in such a manner that, with regard to the temperature (T1) of the above hydrogen storage metal alloy during the stage of hydrogen absorption, the temperature of the said alloy during at least one period during the release of hydrogen is made higher (T2) than the temperature (T1) thereof during the above hydrogen-absorbing process.
Such characteristics lead to the following: during the hydrogen release the temperature (T2) of the aforementioned hydrogen storage metal alloy can be made higher than the temperature (T1) during the hydrogen-absorbing process whereby it is now possible to take out as a utilizable hydrogen the occluded hydrogen at the low-pressure plateau region or at the lower plateau region of the inclined plateau, said occluded hydrogen which has been neither desorbed from the hydrogen storage metal alloy nor utilized before, and to increase electric energy obtained by the fuel battery cell.
For the hydrogen fuel battery of the present invention, it is preferred that the aforementioned controller is capable of appropriately controlling a pressure, temperature and flow rate of the hydrogen gas supplied from the above-mentioned hydrogen storage tank to the above-mentioned fuel battery cell.
As a result thereof, the pressure, temperature and flow rate of hydrogen gas can be controlled whereby it is possible to control amounts of generated electric energy in the fuel battery cell appropriately depending upon the load and to enhance the utilizing efficiency of the hydrogen used in the said fuel battery cell.
For the hydrogen fuel battery of the present invention, it is preferred that the above-mentioned temperature controlling means is arranged so as to enable the heat discharged from the above-mentioned fuel battery cell or the exhaust gas discharged from the said fuel battery cell to be utilized for the above-mentioned heating.
As a result thereof, the discharged heat or the exhausted heat of the fuel battery cell can be utilized for raising the temperature of the above-mentioned hydrogen storage metal alloy whereby no electric energy or the like is necessary for raising the temperature of such a hydrogen storage metal alloy and the efficiency throughout the hydrogen fuel battery can be enhanced.